1. Field of the Invention
The present invention relates to a sheet feeding apparatus equipped with a multiple feed detecting section for detecting the feed of multiple sheets by using ultrasonic waves.
2. Description of Related Art
In the sheet feeding apparatus such as a copying machine and a multifunction peripheral (MFP), optical means have largely been used to detect the feed of multiple sheets of paper. Ultrasonic waves have been used in a facsimile machine and scanner as a new method in recent years.
For example, to improve the detecting accuracy of a multiple feed detecting device, such a device has been proposed that is capable of ensuring stable multiple sheet feed detection, wherein the intensity in the received signals of the ultrasonic wave is very small. Such a device is described, for example, in Japanese Utility Model Publication Jitsukaihei 5-56851.
In this case, the moving surface of an object to be detected is tilted with respect to the surface perpendicular to an ultrasonic wave propagation shaft composed of an ultrasonic transmitter and an ultrasonic wave receiver. Here, part of the ultrasonic wave emitted from the ultrasonic transmitter passes through the object to be detected, as in the conventional arrangement. Further, in this new arrangement, the ultrasonic wave reflected from the surface of the object to be detected travels in a different direction without going back to the transmitter. Thus, the ultrasonic wave does not form a standing wave between the transmitter and receiver, and the amount of the ultrasonic wave received by the receiver is kept almost constant, independently of the position, tilt angle or surface state of the object to be detected. This provides a very stable intensity of the signal received by the receiver, and ensures accurate detection of the feed of multiple sheets of paper, without the need of averaging the received signals.
Other types of multiple sheet feed detecting devices capable of improving the multiple sheet feed detecting accuracy by ultrasonic waves have also been proposed. One such example is the multiple sheet feed detecting device of Japanese Patent Publication Tokkai 2001-199595. In this case, two or more sheets of paper for multiple feed are deformed to provide a clearance area between them. The attenuation of the ultrasonic wave signal is improved by a layer of air in the clearance area, thus causing an improvement in the multiple feed detection accuracy.
When the ultrasonic wave type of multiple feed detecting devices, such as those described above, are used in the copying machine or MFP, however, the temperature will rise due to the fixing heat inside the copying or other machine. The temperature inside the machine will then subsequently decrease at the installation site after the machine has stopped operating. As a result, the transmission and reception sensitivity of the sensor will be reduced due to a drastic ambient temperature change and the temperature characteristics of the ultrasonic sensor itself. Further, when the sensitivity falls, the sensor may incorrectly determine that multiple sheets have been fed when only one has been fed. A decrease in the sensitivity of the sensor may become more problematic when the sensor is continuously driven.
For the reasons stated above, a multiple feed detecting device is currently used on the paper ejection part of a copying machine and multifunction machine. This arrangement, however, has a disadvantage in that a large volume of wasted paper is generated since detection of a multiple feed situation is done after the image has already been formed.
In view of the prior art described above, it is an object of the present invention to provide a novel sheet feeding apparatus equipped with a multiple feed detecting section for detecting the feed of multiple sheets by using ultrasonic waves. Further to this, it is an object of the present invention to provide a sheet feeding apparatus capable of ensuring stable multiple sheet feed detection with stable detection and accuracy. Moreover, it is an object of the invention to detect multiple sheet feeding even when the multiple detecting sensor itself or the installation site thereof, such as, for example, inside the copying machine experiences a drastic temperature change. It is an object of the invention that the sensitivity of the apparatus remains stable during temperature changes.